In a plasma processing apparatus, it is typical that a parallel plate type electrode arranged above a target substrate is provided with a plurality of gas injecting holes. An etching gas is injected through the gas injecting holes toward the whole target substrate and is converted into plasma, thereby simultaneously etching the entire surface of the target substrate.
FIG. 10 is a view schematically showing a conventional plasma processing apparatus. The plasma processing apparatus includes a vacuum chamber 1 in which an upper electrode 22 having gas injecting holes and a lower electrode 2 serving as a substrate mounting table are placed one above the other. A focus ring 5 made of, e.g., silicon, is provided in such a fashion as to surround a target substrate, i.e., a semiconductor wafer (hereinafter referred to as a wafer) 15 mounted on the lower electrode 2.
The wafer 15 is electrostatically attracted by an electrostatic chuck 16. Within the electrostatic chuck 16, there is installed a flat internal electrode 17 to which a chuck voltage is fed from a power supply (not shown). A processing gas selected depending on the kind of processing is injected through the gas injecting holes of the upper electrode 21 toward the wafer 15. A vacuum pump (not shown) performs vacuum evacuation and maintains the pressure inside the chamber 1 at a predetermined level. If a high frequency voltage is applied from a high frequency power supply 12 between the upper electrode 22 and the lower electrode 2, the processing gas is converted into plasma whereby the wafer 15 as a target substrate is subjected to specified processing, e.g., etching.
In the etching process, shapes such as trenches or holes are formed on the wafer in the vertical direction. For the vertical shape formation, a bias voltage is usually supplied to the wafer by applying high frequency voltage in a relatively low frequency thereto. Electric fields perpendicular to the wafer surface are generated by the bias voltage. The vertical shape formation can be performed by the behavior of ions accelerated by the electric fields. Since the electric fields are distorted in an edge portion of the wafer, however, there is posed a problem that the bias voltage is not normally applied, causing shapes to be inclined.
As a result, it is sometimes the case that the devices obtained from a peripheral portion of the wafer 15 have low production yield. The low production yield due to non-uniform etching becomes significant as the diameter of the wafer 15 increases.
In order to cope with such a problem, the focus ring 5 of annular shape is arranged around the wafer 15 placed on the lower electrode 2 serving as a substrate mounting table. Thus the diameter of the wafer 15 in appearance is increased by the focus ring 5. Consequently, the peripheral portion of the wafer 15 is expanded to the peripheral portion of the focus ring 5, and the peripheral portion of the focus ring 5 can be regarded as the peripheral portion of the wafer 15. This assists in making uniform the in-plane etching rate of the wafer 15.
Inasmuch as ions impinge against the focus ring 5 during the plasma processing, the focus ring 5 is heated to an elevated temperature. In case where a plurality of workpieces is continuously processed by plasma, the temperature of the focus ring 5 begins to increase as the first workpiece is subjected to plasma processing. After an arbitrary number of workpieces have been processed, the temperature of the focus ring 5 is stabilized at a constant value, thus coming into a steady state. In other words, the temperature of the focus ring 5 is changed from the outset of processing and then stabilized. Such a change in temperature affects the density of radicals present around the wafer 15, thus causing non-uniform of etching in the peripheral edge portion of the wafer 15.
In order to optimize the state of radical-containing plasma within the chamber, it is very important that the temperature of the members used in the chamber, e.g., the temperature of the focus ring, is optimally controlled depending on the kind of processing in the course of plasma processing.
In view of the above, an attempt is made in Japanese Patent Laid-open Publication No. 2005-353812 (JP2005-353812A) to optimize the plasma state by controlling the temperature of a focus ring to become 50° C. or more greater than the temperature of a wafer when the wafer mounted on a mounting table is processed by plasma. As set forth in JP2005-353812A, however, it is not easy to bury heaters in all of the members used in plasma processing. Furthermore, the focus ring is worn out by the physical impact of plasma or the chemical reaction, which leads to increased cost.
In Japanese Patent Laid-open Publication No. 7-310187 (JP7-310187A), the pressure of a gas filled between a protector plate, i.e., a focus ring, and a mounting table is made adjustable and the temperature of the protector plate is controlled so as to stabilize the plasma processing state. The protector plate is coated with a heating element (or a heater) so that the temperature of the protector plate can be uniformly increased by ion collision. As mentioned in JP7-310187A, however, the protector plate coated with the heating element requires a specified time until the temperature thereof is increased up to a stable state by plasma impact.
In Japanese Patent Laid-open Publication No. 2001-274142 (JP2001-274142A), a silicon ring as a focus ring is provided with a current-feeding portion and is self-heated as an electric current is supplied thereto through the current-feeding portion. The temperature of the silicon ring is controlled by the self-heating. As stated in JP2001-274142A, however, a heater pattern needs to be formed in the silicon ring in order that the silicon ring can be uniformly heated by the self-heating. It is not easy to form the heater pattern. In addition, the technique disclosed in JP2001-274142A poses a problem in that it is impossible to detect the temperature of the self-heating silicon ring and to optimally control the temperature of the silicon ring depending on the condition of plasma processing.